| Short-burst communication system is widely used in satellite communications,military communications,deep space exploration and high-speed mobile communications and so on.Continuous Phase Modulation(CPM)with the characteristics of high spectral efficiency and power efficiency is a popular waveform in these systems.Shaped-Offset Quadrature Phase Shift Keying(SOQPSK)is a special kind of CPM modulation,which has both the characteristics of CPM and OQPSK,and has the characteristics of strict constant envelope.It has superior performance in frequency utilization and power utilization.Considering the wireless channels with extremely low SNR,limited power and bandwidth,and large Doppler frequency offset,it is in urgent need for short-burst communication to solve the problem that how to use the limited power and frequency efficiently and reasonably,to achieve high reliability for carrier synchronization communications.In the first part of this paper,we discuss the background and the research status at home and abroad.In the second part,we study the basic concept of CPM and the characteristics of the CPM signals,and then discuss the modulation and demodulation algorithm of a special kind of CPM signals,SOQPSK signal.We mainly research on non-recursive trellis signal modulation and Max-Log-Map demodulation of SOQPSK.Then the pilot aided carrier synchronization algorithm is studied,and simulation analysis for the carrier synchronization algorithm of frequency domain and time domain is done.From simulation results,we can find that the frequency domain estimation algorithm has large estimation range,but the estimation accuracy is limited by pilot overhead and frequency resolution.So in order to get higher estimation precision,the complexity must be much higher.However,the estimation precision of the time domain estimation algorithm is relatively high,and the complexity of it is much lower compared with the frequency domain estimation algorithm.Finally,the data frame structure is studied,and then symmetrical data frame structure is put forward in this paper.We called it symmetrical frame structure.Due to the time symmetrical characteristic,the frequency estimation and the phase estimation can be decoupled,and the influence of additional phase of the frequency offset estimation on the phase estimation can be reduced,which can make the frequency offset estimation and phase estimation are independent of each other.In the third part,a joint time-frequency domain carrier synchronization scheme under the extremely low SNR condition is proposed in this paper.Firstly,TPC coding is conducted on the input signal,and the encoded sequence is then spectrum spread.Because of the gain produced by spread spectrum,it is suitable for communication in extremely low signal-to-noise ratio.After the spread spectrum sequence and pilot frequency are remutiplexed,the SOQPSK modulation is conducted.At the receiver,the received signals are demultiplexed into pilot and data.RPA algorithm and correlation algorithm using pilot sequence are adopted during coarse estimation,and the correlation algorithm using pilot sequence and data is adopted during fine estimation.After the signal is demodulated,expanded and decoded,it is encoded,spread spectrum and modulated again,and then the frequency offset is estimated by the cross-correlation algorithm to improve the estimation accuracy.The scheme proposed in this paper can realize more efficient synchronization in extremely low SNR environment,and when taking estimation range and estimation precision into consideration at the same time,the complexity is relatively low.Finally,in the Quartus ? 9.0 development environment,carrier synchronization algorithm is implemented by FPGA which is based on Verilog hardware description language.Based on the implementation of algorithm,information processing speed and resource usage of the FPGA design are analyzed,and the correctness of the scheme is verified through the waveform simulation. |
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